This invention relates to scavenging trace amounts of chlorine in ethylene dichloride ("EDC") generated by the reaction of chlorine and ethylene. This reaction is the basis for the well-known "direct chlorination of ethylene" in the widely used commercial catalytic process for the production of EDC. The reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor whether the reaction is carried out with a slight excess of ethylene, or alternatively chlorine, fed to the reactor. The heat of reaction is dissipated either through conventional water cooling of a typical low temperature direct chlorination reactor operating in the range from about 50.degree. C. to about 65.degree. C., or, by operating the reactor at the boiling point of EDC under pressure up to about 50 psig, hence referred to as a "boiling reactor". The boiling reactor is a particular type of direct chlorination reactor.
Though it has long been known that the effluent from a boiling reactor is highly corrosive, it was only recently found that the main cause of such corrosion was the presence of free chlorine and trace quantities of moisture. We do not know of any prior art directed to minimizing the concentration of free chlorine in the vent gases from a boiling reactor.
We are well aware that the problem of minimizing corrosion due to the effluent, without specific regard as to minimizing the production of free chlorine, has confronted many persons skilled in the art. Corrosion is pronounced even at room temperature; it gets exponentially worse, doubling for every 10.degree. C. increase, so that in the range above 50.degree. C. it is in full effect; and, if one wishes to operate a commercial boiling reactor, one cannot avoid operating in the elevated temperature range. To minimize corrosion in such a manner as to provide an effluent which is not only acceptably corrosive but economically not unduly burdensome is a difficult problem to which a better solution is constantly sought. Part of the difficult lies with the varied considerations which define the problem, as it presents itself in different guises, hence the elusiveness of the solution; and by no means a minor part lies in the unforgiving economics of any solution to the problem. It is axiomatic that solutions to industrial problems must be economically acceptable.
As is well known, the economics of chemical engineering unit operations in the production of EDC of vinyl chloride monomer are such that, optimally, the ethylene and chlorine are converted to EDC without the formation of unwanted byproducts and most important, without leaving any free chlorine residue in the effluent. The problem of corrosion is discussed in "Alloy Selection for VCM Plants" of Schillmoller, C. M., Hydrocarbon Processing pg 89-93, March 1979.
In practice the reaction is controlled so that carbon steel equipment may be used. The problem is that as little as from about 100 parts per million (ppm) to about 5000 ppm (0.5%) by weight (by wt) of free chlorine, has a highly corrosive effect on downstream equipment when trace amounts of water are present in the effluent as a contaminant. The problem has been countered in the past by washing and neutralizing the effluent with aqueous alkali but this generates an alkali metal salt. Further complications arise due to poisoning of catalyst due to the presence of alkali in subsequent processing steps, and to the relatively high solubility of halogenated hydrocarbons in the alkali wash, which complication is exacerbated by the large volume of the wash required to be circulated. Then to dry the washed EDC was an undesirable cost.
From the viewpoint of fluid handling, not to mention energy utilization, the solution to the problem was economically onerous and technically inelegant. It was decided to concentrate on removal of chlorine without alkali washing the reaction products, and to do so as soon after they leave the direct chlorination reactor as possible. Further, it was decided not to be concerned with the entrainment of ferric chloride (FeCl.sub.3) catalyst, relying on controlling reaction conditions sufficiently to entrain only a minimal and acceptable level in the effluent from the boiling reactor.
It is kown that ethylene chloride (EDC) reacts with chlorine to produce 1,1,2-trichloroethane ("triane") in the presence of ultraviolet ("u-v") light as taught in U.S. Pat. No. 2,174,737, when from 0.2 to about 1.0 mol of chlorine per mol of EDC is present in the liquid phase. However, not only is this reaction inhibited by the presence of oxygen, it requires that a relatively large molar amount of chlorine relative to ethylene be present if the reaction is to produce triane.
In the boiling reactor, whether it is run with a slight excess of chlorine or ethylene, the main byproduct is triane which probably forms through radical reactions beginning with homolytic dissociation of a small fraction of the chlorine. However, oxygen which is deliberately introduced as an impurity with the chlorine, tends to increase selectivity to EDC by inhibition of free radical reactions that produce triane (see Kirk & Othmer, supra).
Because the formation of triane is deemed undesirable in the direct chlorination of ethylene where EDC is the desired product, its formation is suppressed by the oxygen, usually supplied as injected air (see Encyclopedia of Chemical Technology, Kirk & Othmer, Third Edition, Vol. 23, pg 871, Wiley Interscience 1984).
Since the concentration of free chlorine and ethylene are each so low relative to the EDC generated, whether in the liquid phase of a low temperature reactor, or in the effluent vapor phase as the reaction products leave the boiling reactor, or in the liquid phase after the effluent is condensed, it seemed unlikely that the chlorine and ethylene would react. It would simply be difficult for the chlorine and ethylene molecules to find each other with all the EDC present. It just did not occur to us to tray and react the free chlorine and EDC to produce triane, the formation of which we were deliberately suppressing. Moreover, the amount of oxygen present is relatively high in relation to the chlorine and ethylene and would likely inhibit the production of triane.